Network Working Group L. Gong
Internet Draft W. Cheng
Intended status: Standards Track China Mobile
Expires: October 8, 2022 C. Lin
M. Chen
New H3C Technologies
R. Chen
ZTE Corporation
April 8, 2022
Advertising Exclusive Links for Flex-Algorithm in IGP
draft-gong-lsr-exclusive-link-for-flex-algo-00
Abstract
This document proposes a method to advertise exclusive links for
Flex-Algorithm in IGP.
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document authors. All rights reserved.
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Table of Contents
1. Introduction ................................................ 2
1.1. Requirements Language .................................. 2
2. Problem Statement ........................................... 3
3. New Extension to Prune Links ................................ 3
4. Backward Compatibility Method in IS-IS ...................... 5
5. Security Considerations ..................................... 5
6. IANA Considerations ......................................... 5
7. References .................................................. 5
7.1. Normative References ................................... 5
7.2. Informative References ................................. 5
8. Acknowledgments ............................................. 6
Authors' Addresses ............................................. 7
1. Introduction
Flexible Algorithm (Flex-Algorithm) allows IGP to compute
constraint-based paths. [I-D.ietf-lsr-flex-algo] specifies the usage
of Flex-Algorithm in Segment Routing (SR) data planes - SR MPLS and
SRv6. [I-D.ietf-lsr-ip-flexalgo] extends the Flex-Algorithm for
native IPv4 and IPv6 data planes.
In some scenarios, exclusive links may be deployed for Flex-
Algorithm, but not for best-effort service. However, these links
cannot be pruned in normal SPF calculation, and unexpected flows may
be steered into these links.
This document proposes a method to advertise exclusive links for
Flex-Algorithm in IGP.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
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BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Problem Statement
Flex-Algorithm allows IGP to compute the best paths along the
constrained topology.
A network topology is shown in Figure 1. Node A, B, C and D have an
extra link between each other. These links have EAG attribute of
"red" color.
Flex-Algorithm 128 are enable on Node A, B, C and D, with metric-
type of IGP cost and EAG rule of including "red". The topology used
by Flex-Algorithm 128 is shown in Figure 2.
Flex-Algorithm 128 are used to transmit particular flows, such as
network slice. The links used by Flex-Algorithm 128 are sub-
interfaces with dedicated queues for bandwidth guarantee. So it is
expected that only the particular flows are transmitted on these
links using Flex-Algorithm 128. However, these links are also
contained in the default topology used by normal SPF calculation,
and unexpected flows of best-effort service may be steered into
these links. Therefore, it is a problem that exclusive links for
Flex-Algorithm cannot be pruned in normal SPF calculation.
A======C------E
|| || |
|| || |
|| || |
B======D------F
Figure 1
A------C
| |
| |
| |
B------D
Figure 2
3. New Extension to Prune Links
A new Link Flags sub-TLV is defined in IS-IS. The format is as the
following:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Type: TBD.
o Length: Variable, dependent on the size of the Flags field. MUST
be a multiple of 4 octets.
o Flags: Following flags are currently defined.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|P| ~
+-+-+-+-+-+-+-+-+
o P-Flag: Pruning the link during SPF calculation.
The Link Flags sub-TLV is advertised in the TLVs/sub-TLVs below:
o TLV-22 (Extended IS reachability) [RFC5305]
o TLV-222 (MT-ISN) [RFC5120]
o TLV-23 (IS Neighbor Attribute) [RFC5311]
o TLV-223 (MT IS Neighbor Attribute) [RFC5311]
The Link Flags sub-TLV with P-Flag can be advertised for the
exclusive links used by Flex-Algorithm, so that these links will be
pruned during normal SPF calculation.
When using the Link Flags sub-TLV, all nodes in the same area or
level must support this sub-TLV. Otherwise routing loops may be
caused by topology inconsistences.
Relevant extensions for OSPF will be included in the future version
of this draft.
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4. Backward Compatibility Method in IS-IS
As specified in [RFC5305], if a link is advertised with the maximum
link metric (2^24 - 1), this link MUST NOT be considered during the
normal SPF computation in IS-IS.
Instead of advertising the Link Flags sub-TLV in Section 3, the
metrics of exclusive links for Flex-Algorithm may be advertised by
the value of (2^24 - 1). It can also lead to the pruning of these
links in normal SPF computation.
If the associated Flex-Algorithm needs to use IGP-Cost as its
metric-type in path calculation, the Flex-Algorithm-associated
Generic Metric sub-TLV defined in [I-D.cheng-lsr-flex-algo-metric]
can be advertised to carry the actual value.
5. Security Considerations
TBD
6. IANA Considerations
Link Flags sub-TLV (TBD)
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, May 2017
[I-D.ietf-lsr-flex-algo] Psenak, P., Hegde, S., Filsfils, C.,
Talaulikar, K., and A. Gulko, "IGP Flexible Algorithm",
draft-ietf-lsr-flex-algo-18 (work in progress), October
2021.
7.2. Informative References
[I-D.ietf-lsr-ip-flexalgo] Britto, W., Hegde, S., Kaneriya, P.,
Shetty, R., Bonica, R., and P. Psenak, "IGP Flexible
Algorithms (Flex- Algorithm) In IP Networks", draft-ietf-
lsr-ip-flexalgo-04 (work in progress), December 2021.
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[RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
Topology (MT) Routing in Intermediate System to
Intermediate Systems (IS-ISs)", RFC 5120, DOI
10.17487/RFC5120, February 2008, <https://www.rfc-
editor.org/info/rfc5120>.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, DOI 10.17487/RFC5305, October
2008, <https://www.rfc-editor.org/info/rfc5305>.
[RFC5311] McPherson, D., Ed., Ginsberg, L., Previdi, S., and M.
Shand, "Simplified Extension of Link State PDU (LSP) Space
for IS-IS", RFC 5311, DOI 10.17487/RFC5311, February 2009,
<https://www.rfc-editor.org/info/rfc5311>.
[I-D.cheng-lsr-flex-algo-metric] Cheng, W, "Advertisement of
Dedicated Metric for Flexible Algorithm in IGP", draft-
cheng-lsr-flex-algo-metric-00 (work in progress), February
2022.
8. Acknowledgments
The authors would like to thank the following for their valuable
contributions of this document:
TBD
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Authors' Addresses
Liyan Gong
China Mobile
Email: gongliyan@chinamobile.com
Weiqiang Cheng
China Mobile
Email: chengweiqiang@chinamobile.com
Changwang Lin
New H3C Technologies
Email: linchangwang.04414@h3c.com
Mengxiao Chen
New H3C Technologies
Email: chen.mengxiao@h3c.com
Ran Chen
ZTE Corporation
Email: chen.ran@zte.com.cn
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